First discovery of Pravitoceras sigmoidale Yabe from the Yezo Supergroup in Hokkaido, Japan

An almost intact specimen of an uppermost Cretaceous heteromorph ammonoid, Pravitoceras sigmoidale Yabe, was recently discovered from the Hakobuchi Formation of the Yezo Supergroup in the Hidaka area, Hokkaido. The specimen is preserved in a lenticular calcareous nodule. It retains two whorls of planispiral phragmocone and a succeeding retroversal hook, which is associated with a lower jaw apparatus near the aperture. P. sigmoidale occurs within the Inoceramus shikotanensis Zone at the Hidaka section. Patagiosites alaskensis, Gaudryceras sp., Inoceramus shikotanensis, and Gigantocapulus problematicus cooccur with P. sigmoidale. The megafossil assemblage resembles those of the Upper Campanian-Lower Maastrichtian in the Sakhalin area. P. sigmoidale has a very short range assignable to the Upper Campanian. However, it has been assumed to be an endemic species of the Izumi Group in southwest Japan, and has never before been found from northeast Japan. Scarcity of common zone-indexing taxa is a bottleneck for biostratigraphic research of the uppermost Cretaceous System in Japan. The first discovery of P. sigmoidale from the Hidaka area is a significant step for much more precise biostratigraphic correlation between the Yezo Supergroup and the Izumi Group.